This invention relates to the preparation of coated aluminum-alloy articles, and, more particularly, to the preparation of coated aluminum rivets.
Fasteners are used to mechanically join the various structural elements and subassemblies of aircraft. For example, a large transport aircraft typically includes over one million fasteners such as bolts, screws, and rivets. The fasteners are formed of strong alloys such as titanium alloys, steel, and aluminum alloys. In some cases, the fasteners are heat-treated, as by a precipitation-hardening aging treatment, to achieve as high a strength, in combination with other desirable properties, as is reasonably possible for that particular alloy. Heat-treating usually involves a sequence of one or more steps of controlled heating in a controlled atmosphere, maintenance at temperature for a period of time, and controlled cooling. These steps are selected for each particular material in order to achieve its desired physical and mechanical properties. In other cases, the fastener is used in an as-worked condition.
It has been the practice to coat some types of fasteners with organic coatings to protect the base metal of the fasteners against corrosion damage. In the usual approach, the fastener is first fabricated and then heat-treated to its required strength. After heat-treatment, the fastener is etched with a caustic soda bath to remove the scale produced in the heat-treatment. Optionally, the fastener is alodined or anodized. The coating material, dissolved in a volatile carrier liquid, is applied to the fastener by spraying, dipping, or the like. The carrier liquid is evaporated. The coated fastener is heated to elevated temperature for a period of time to cure the coating. The finished fastener is used in the fabrication of the structure.
This coating approach works well with fasteners made of a base metal having a high melting point, such as fasteners made of steel or titanium alloys. Such fasteners are heat-treated at temperatures well above the curing temperature of the coating. Consequently, the curing of the coating, conducted after heat-treating of the fastener is complete, does not adversely affect the properties of the already-treated base metal.
On the other hand, aluminum alloys have a much lower melting point, and thence a generally much lower heat-treatment temperature, than steel and titanium alloys. It has not been the practice to coat high-strength aluminum-alloy fasteners with curable coatings, because it is observed that the curing treatment for the coating can adversely affect the strength of the fastener. The aluminum-alloy fasteners are therefore more susceptible to corrosion than would otherwise be the case. Additionally, the presence of the organic coating aids in the installation of the fastener for titanium alloys and steel. The absence of the coating means that aluminum fasteners such as rivets must be installed using a wet sealant compound for purposes of corrosion protection. The wet sealant compound typically contains toxic components and therefore requires precautions for the protection of the personnel using it and for environmental protection. It is also messy and difficult to work with, and may require extensive cleanup of the area around the fastener using caustic chemical solutions.
There exists a need for an improved approach to the protection of aluminum-based fasteners such as rivets. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing an aluminum-alloy article such as a fastener, and more specifically a rivet. For a heat-treatable article, the article is heat-treated to have good mechanical properties and also is protected by a cured organic coating. For a cold-worked article, the coating is applied and cured while still achieving the desired deformation state in the article. The application of the coating does not adversely affect the desired final properties of the article. The present approach is accomplished at an additional cost of much less than one cent per fastener above its unprotected cost.
In accordance with the invention, a method for preparing an aluminum-alloy article comprises the steps of providing an aluminum-alloy article precursor that is not in its final heat-treated state, and anodizing the article precursor, preferably in chromic acid solution and also preferably without sealing the surface of the article precursor in the anodizing step. The method further includes providing a curable organic coating material, the coating material having a non-volatile portion that is predominantly organic and is curable at about a heat-treatment temperature of the aluminum-alloy article precursor, and applying the organic coating material to the aluminum-alloy article precursor. The coated aluminum article precursor is heat treated to its final heat-treated state at the heat-treatment temperature and for a time sufficient to complete the heat treatment of the aluminum alloy precursor and to cure the organic coating, forming the article.
In one embodiment of the present approach, the article precursor and thence the article is made of an aluminum alloy having a temper achieved by artificial aging to its final state. This article precursor is provided in a solution treated/annealed condition suitable for the subsequent utilization of the strengthening heat-treatment, but not as yet final heat-treated. The article precursor is anodized, preferably in chromic acid solution, to improve the adhesion of the subsequently applied coating to the article precursor, and also preferably without sealing the surface of the article precursor. The organic coating material, preferably dissolved in a suitable carrier liquid, is applied to the anodized surface of the article which is not in its final heat-treated state. The carrier liquid is removed by evaporation. The heat treatment of the article precursor is thereafter completed to bring the article to its full strength by heating to elevated temperature in a precipitation-hardening aging treatment. During the precipitation-hardening aging treatment according to the combination of temperature(s), time(s), and environment(s) specified for the aluminum-alloy base metal of the fastener, the coating is cured. Thus, no separate curing procedure is required after coating an already heat-treated article, which curing procedure would be likely to adversely affect the strength of the base metal of the article.
In another embodiment, the article is made of an aluminum alloy having a temper that is achieved by natural aging. (The distinction between artificial and natural aging is that during precipitation treatment artificial aging involves heating the article to elevated temperature, and natural aging is accomplished at room temperature.) In this case, the article is deformed prior to coating with the organic coating material and naturally aged. It is coated and heated to accomplish curing of the coating and some artificial aging. Absent the additional deformation during fabrication and prior to curing of the coating, the article is found to overage when heated to cure the coating.
In yet another embodiment, the article is not normally heat treated, but instead is used in a final deformation state that imparts significant cold work to its structure, either before or during fabrication. In this case, the article precursor is over-deformed to a deformation state greater than that required in the final article, optionally anodized in chromic acid solution, coated with the organic coating material, and then heated to cure the coating and partially anneal the article precursor to the required deformation state.
All of these embodiments yield surprising and unexpected technical and cost advantages when used in conjunction with high-strength aluminum fasteners such as rivets. The aluminum-alloy fasteners exhibit their full required strength produced by the heat-treatment used by itself or the required deformation state. The achieving of a specified strength level is important, because users of the rivets, such as the customers of aircraft, will not permit a sacrifice of mechanical performance to achieve improved corrosion resistance. Instead, in the past they have required both acceptable mechanical performance and also the use of wet sealants to achieve acceptable corrosion resistance. In the present approach, on the other hand, the article has both acceptable mechanical performance and a coating for acceptable corrosion protection. Therefore, during installation of a fastener made by the present approach, wet sealants need not be applied to the fastener and faying surfaces of the hole into which the fastener is inserted just before upsetting the fastener.
The elimination of the requirement for the wet sealant installation approach for the over-700,000 rivets in a large cargo aircraft offers a cost savings of several million dollars per aircraft. The elimination of the use of wet sealants also improves the workmanship in the fastener installation, as there is no possibility of missing some of the fasteners as the wet sealant is applied. The coated fasteners are more resistant to corrosion during service than are uncoated fasteners.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.